1. Field of the Invention
The present invention relates to a filter circuit and a radio communication system comprising a filter, and in particular, to an MEMS filter circuit that limits a frequency band and a wireless device comprising this MEMS filter.
2. Description of the Related Art
A known filter circuit has resonance circuits cascaded (connected together in series) between an input terminal and an output terminal as disclosed in James Brank et al., “RF MEMS-based tunable filters,” International Journal of RF and Microwave Computer-Aided Engineering, Vol. 11, Issue 5, pp. 276-284, September 2001. In general, an equivalent circuit such as a resonance circuit is formed by an inductor L and a capacitor C, and a resistor is added to the equivalent circuit taking a possible loss in the filter circuit into account. The resonance frequency of a resonance circuit without any resistor is given by:f0=1/sqrt(L*C)
where L and C denote the inductance and capacitance, respectively, of the resonance circuit.
For a filter circuit composed of cascaded resonance circuits, the passing frequency range and inhibition area attenuation can be determined by appropriately setting values for the coupling coefficients (m2 and m3) of resonators which represent the amount of coupling among the resonance circuits and for external Qs (m1 and m4) representing the amounts by which an input and output units excite the resonance circuit.
An actual filter circuit is composed of a microwave circuit, a distributed constant circuit, or a lumped constant circuit. A resonance circuit composed of a microwave circuit has a filter composed of a metal cavity or a dielectric placed in a metal cylinder. A filter circuit composed of a distributed constant circuit is composed of a resonance circuit made of microstrip lines or two-dimensional wires. A filter circuit composed of a lumped constant circuit is composed of a constant circuit such as an inductor or a capacitor. In general, the size of the filter may decrease in order of the microwave circuit, distributed constant circuit, or lumped constant circuit. However, the amount of loss in the filter circuit increases with decreasing filter size. In a filter circuit composed of cascaded resonators, energy must pass through all resonance circuits. Accordingly, to reduce a possible loss, circuit scale is increased to provide a resonance circuit with a reduced loss.
A known method of constructing a filter circuit in which signal energy does not pass through all resonance circuits is parallel connection of resonance circuits disclosed in, for example, The Institute of Electronic Information and Communication Engineers: MW82-54. In a filter circuit thus having resonance circuits arranged in parallel, input power is distributed to each resonance circuit. Signal energy may thus suffer a passing loss in only one resonator, thus making it possible to reduce the loss of the filter as a whole. To allow resonators to be connected together in parallel, filter characteristics are obtained by configuring the resonators so that they have different resonance frequencies and detecting and synthesizing input signals so that the resonators having adjacent resonance frequencies output signals of opposite phases.
However, implementation of such a filter circuit conventionally requires opposite phase detecting means or opposite phase synthesizing means using a delay circuit such as the one disclosed in JPA-2001-345601 (KOKAI). This is because the conventional filter circuit uses a free vibrator to cause resonance. If a filter circuit is made on a semiconductor, it is difficult to build opposite phase detecting means in the filter circuit. Provision of a delay circuit is also disadvantageous in terms of a loss. Further, in addition to the resonance frequencies of the resonators, coupling coefficients need to be varied in order to adjust center frequency or bandwidth included in filter characteristics. A mechanism for varying these parameters is disadvantageously complicated. If a common Chevyshev filter is composed of k (integer) resonators, (2k+1) parameters need to be adjusted. A larger number of parameters need to be adjusted for an elliptic function filter circuit or the like which involves more complicated coupling.
As described above, in a filter circuit composed of cascaded resonance circuits, signals pass through the large number of resonance circuits, resulting in too long a signal path and a heavy loss. Further, to vary the center frequency or bandwidth, included in the filter characteristics, many parameters need to be varied, thus disadvantageously requiring a complicated control mechanism.